The present invention relates to SrTiO.sub.3 -CaTiO.sub.3 -Bi.sub.2 O.sub.3.xTiO.sub.2 type semiconducting ceramic capacitors, particularly the capacitors which are suitable for use in circuits demanding severe temperature properties, such as a coupling or a trap circuit. In according with this invention, the ceramic capacitors are prepared from compositions consisting predominantly of the nonstoichiometric solid solutions which are derived from a mixture composed mainly of strontium titanate (SrTiO.sub.3), calcium titanate (CaTiO.sub.3), bismuth oxide (Bi.sub.2 O.sub.3) and titanium oxide (TiO.sub.2) and in which minor proportions of suitable amounts of at least one member selected from the group consisting of manganese, cobalt, nickel, chromium, vanadium, niobium, tantalum, lanthanum and cerium ions are added thereto. This mixture is then fired in an oxidizing atmosphere to produce a ceramic and the resulting ceramic is heated in a reducing atmosphere and some amount of oxygen eliminated therefrom to produce the nonstoichiometric solid solutions. The resulting nonstoichiometric solid solutions are partially re-oxidized in an oxidizing atmosphere at elevated temperature to produce the semiconducting ceramic capacitors.
Although semiconducting ceramic capacitors are a relatively new field of technology, it is now recognized they they are superior to conventional insulating ceramic capacitors since they exhibit a large capacitance, are small in size and relatively compact, and exhibit other excellent characteristics. Semiconducting ceramic bodies for use as capacitors are classified into two types, viz, the valence control type and the reduction-reoxidation type, according to their composition and the method of their production. The semiconducting ceramic bodies of the valence control type are composed predominantly of barium titanate to which minor amounts of other elements, which have an ionic radius similar to those of the constituents of barium titanate but with a different valency, are added thereto. Since the characteristics of these valence control type semiconductors are strongly affected by the purity of raw materials, the maintenance of said purity during the manufacturing process, and the necessity of accurately weighing the raw materials in order to combine them in suitable proportions, make it difficult, if not impossible, to produce such ceramics on an industrial scale. In fact, it is difficult to prepare such ceramics in the laboratory, let alone on an industrial scale. In addition, the valence control type semiconductors have other defects in that their specific resistivity cannot be lowered below 10 ohm-cm, and their electrical properties are intrinsically fixed so that the temperature dependence of their capacitance cannot be changed arbitrarily.
On the other hand, the capacitors made from ceramics of the reduction-reoxidation type are free of the defects peculiar to the valence control type ceramics but have other difficulties. For example, capacitors of this type, generally have such defects that the insulation resistance shows a sharp fall when the applied voltage is increased and, therefore, their working voltage in usual practical applications is generally 10 volts and the upper limit is fixed at about 12 volts. Another shortcoming of these capacitors is that undesirable changes of the electrical properties occur when lead wires are directly soldered to silver electrodes because it is difficult to stabilize the barrier layers. To prevent this, lead wires are usually attached to the silver electrodes with conductive adhesives. But in practical use, when said capacitors are connected in a circuit, their lead wires are heated to an elevated temperature during the soldering processes, which sometimes leads to damage of the conductive adhesives. Particularly in miniaturized electric circuits, in which lead wires are short, very careful treatment is needed.
In short, the prior semiconducting ceramic capacitors, the main constituent of which is barium titanate (BaTiO.sub.3), possess no electrical properties of barium titanate, that is, its temperature independency of the capacitance and tan .delta., and the high dielectric constant.
Further, because of their low breakdown voltage, these capacitors have been limited in their utility as stated above.